Between the orbits of Mars and Jupiter circles a broad ring of rocky debris - the leftover building blocks of a planet that never formed, because Jupiter’s gravity kept stirring the material too violently for it to clump together. Its largest resident, Ceres, is a dwarf planet in its own right.
More than a million asteroids have been catalogued, and the vast majority orbit here.
Ceres alone holds about a third of the entire belt’s mass.
It is not a destroyed planet - Jupiter’s gravity prevented one from ever forming there.
Mostly empty space
Movie asteroid fields are a myth. The belt holds over a million known asteroids, but they are spread through a volume so colossal that neighbouring rocks sit on average about a million kilometres apart. Every spacecraft that has crossed the belt has flown through without incident - and everything in it combined adds up to less than 4% of the Moon’s mass.
Physical characteristics
Diameter
0 km
Dimensions
2.1 – 3.3 AU from the Sun km
Atmosphere and composition
Composition
Rocky and metallic bodies, from dwarf planet Ceres down to countless pebbles and dust grains.
The trailblazer of the outer Solar System: first spacecraft through the asteroid belt and first to fly past Jupiter, in December 1973. Its last faint signal was heard in January 2003, from roughly 12 billion km away.
DawnNASA · Orbiter · 2007–2018 · completed
Powered by gentle ion engines, Dawn became the only spacecraft ever to orbit two different deep-space worlds: the asteroid Vesta (2011-2012) and the dwarf planet Ceres (2015-2018). It ran out of fuel in 2018 and remains in a stable orbit around Ceres.
LucyNASA · Flyby probe · 2021 · en route
Lucy is on a 12-year tour of the Jupiter Trojan asteroids - primitive leftovers trapped in Jupiter’s orbit - with its main tour beginning in 2027. Practice flybys of the main-belt asteroids Dinkinesh and Donaldjohanson have already delivered surprises.
What is still uncertain
Listed size is the belt’s radial extent from the Sun, not a body diameter.